1. Field of the Invention
The present invention relates to a method of and a system for CO2 capture in oxy-combustion. More particularly, the present invention relates to a two-staged CO2 capturing procedure, which optimizes the CO2 cooling duty and compression power.
2. Description of the Related Art
The flue gases of a conventional power station typically contain from about 4% (by volume) to about 14% carbon dioxide (CO2). It is commonly believed that this CO2 represents a significant factor in increasing the greenhouse effect and global warming. Therefore, there is a clear need for efficient methods of capturing CO2 from flue gases so as to produce a concentrated stream of pressurized CO2 that can readily be transported to a safe storage site or to a further application. CO2 has been captured from gas streams by four main technologies: absorption, where CO2 is selectively absorbed into liquid solvents; membranes, where CO2 is separated by semipermeable plastics or ceramic membranes; adsorption, where CO2 is separated by adsorption on the surfaces of specially designed solid particles; and, low temperature/high pressure processes, where the separation is achieved by condensing the CO2.
At present, the best proven technique to capture CO2 from a flue gas is to scrub the flue gas with an amine solution to absorb CO2 to the solution. This technology has reached the commercial state of operation for CO2 capture systems from small scale flue gases. However, its application decreases considerably the total efficiency of the power plant. Another difficulty is that, in order to minimize contamination of the solvent by impurities, effective measures are needed to clean the flue gas from, for example, sulphur and nitrogen oxides.
Oxy-combustion systems use oxygen, usually produced in an air separation unit (ASU), instead of air, for the combustion of the primary fuel. The oxygen is often mixed with an inert gas, such as recirculated flue gas, in order to keep the combustion temperature at a suitable level. Oxy-combustion processes produce flue gas having CO2, water and O2 as its main constituents, the CO2 concentration being typically greater than about 70% by volume. Therefore, CO2 capture from the flue gas of an oxy-combustion process can be done relatively simply by using refrigerated separation. The water vapor is usually removed from the flue gas of an oxy-combustion process by compressing and cooling the flue gas. Further treatment of the flue gas may be needed to remove air pollutants and non-condensed gases (such as nitrogen) from the flue gas before the CO2 is separated to be sent to storage.
U.S. Pat. No. 6,898,936 discloses an oxy-combustion process with flue gas recirculation, in which a portion of the flue gas is compressed in several steps to a very high pressure of about 345 bar, and cooled to below the critical temperature, 31.05° C. of CO2 to condense the CO2 from the flue gas. The resulting gas stream, which is O2 rich, is expanded back to about 138 bar and conducted to an air separation unit.
U.S. Pat. No. 6,574,962 discloses an oxy-combustion process with flue gas recirculation, in which a portion of the flue gas is cooled in several steps to a very low temperature, ranging from about −51° C. to about −12° C., and compressed to above about 5.8 bar to condense CO2 from the flue gas. The resulting gas stream, rich in O2, is combined with an O2 rich gas stream from an air separation unit and conducted as combustion gas to the combustion furnace.
The above-mentioned prior art solutions are complicated by using either a very high pressure or a very low temperature to capture CO2. Therefore, there still exists a need for a more simple and an economic process to capture CO2 from the flue gas of the oxy-combustion of carbonaceous fuels.